1. Field of the Invention
This invention relates to a method and an apparatus for equalizing received transmission signals containing at least a sync word and arbitrary data in each slot such as in a time division multiple access (TDMA) system.
2. Description of the Prior Art
Among available digital communication systems for transmitting signals containing at least a sync word and arbitrary data in each slot are time division multiple access (TDMA) systems in which a large number of stations may transmit signals with the same carrier frequency to a relay unit for mutual communication so that signals do not temporally overlap each other. That is, a TDMA frame (a predetermined time length), which is a basic period for signal transmission and reception, is set and communication with a called station is carried out using an allocated pair of time positions (time slots) within the TDMA frame. Therefore, it is necessary for each station to transmit signals in an allocated time slot within the frame and to effect time position control (burst synchronization control) so that the transmitted signals do not "collide" with other transmitted signals. It is also necessary for each station to obey the time reference of a reference station to control the time position of transmitted burst signals so that burst signals transmitted by the stations do not collide against one another. In general, such burst synchronization control and compression and expansion of the transmission signals at the transmitting station and at the receiving station, respectively, are characteristic of TDMA communication.
Among applications of the above-described TDMA system are car telephone systems.
As shown in FIG. 1, in car telephone systems, each frame contains six slots for a frequency band. Each frame is made up of 1944 bits (972 symbols) and is 40 ms long. In other words, there are 25 frames per second.
Each slot in each frame has a format consisting of a 6-bit guard time (G), a 16-bit data slot, a 28-bit (14-symbol) sync word, a 122-bit data slot, a 12-bit slow associated control channel (SACCH) and a coded digital verification color code (CDVCC), as shown at A in FIG. 2, or a format consisting of a 28-bit (14-symbol) sync word, a 12-bit SACCH, a 130-bit data slot, a 12-bit CDVCC, a 130-bit data slot and 12-bit reserved slot for bits 00 . . . 00 (RSVD), as shown at B in FIG. 2. These formats are slot formats used when transmitting from a base station to a car telephone terminal.
Meanwhile, when receiving the slots transmitted from the car telephone system, an equalization process is usually carried out for signals received in each slot.
For equalizing a reception slot, that is of a current slot, of the slot format used when transmitting from a base station to a car telephone terminal, as shown at B in FIG. 2, the processing of equalization is started at the sync word which is positioned at the leading end of the reception slot. On the other hand, 162 symbols per reception slot means a reception slot period of approximately 6.7 ms which is fairly long with respect to fading at a vehicle speed of approximately 100 km/h. Under such conditions, the transmission channel characteristics may fluctuate considerably from one reception slot to another.
Therefore, should any errors be present somewhere within the reception slot, such as in its data area, it may frequently occur that the equalization process cannot be carried out downstream of the error site, so that the risk is high that data downstream of the error site will be lost in its entirety.
Because, under such circumstances, information for identifying the transmission channel cannot be correctly obtained, the characteristics of the subsequent operations may deteriorate.